The present invention relates to fiber optic couplers and more particularly to a low-cost method of making fiber optic couplers.
Certain types of fiber optic systems require couplers in which at least a portion of the light propagating in one fiber is coupled to one or more other fibers.
It has been known that coupling occurs between two closely spaced cores in a multiple core device. The coupling efficiency increases with decreasing core separation and, in the case of single-mode cores, with decreasing core diameter. There have been developed a number of couplers that are based on these principles, and which are capable of low loss operation, i.e. they exhibit a loss of about 1 dB or less.
Multimode and single-mode couplers have been formed by positioning a plurality of fibers side-by-side along a suitable length thereof and fusing the claddings together to secure the fibers and reduce the spacings between the cores. Coupling can be enhanced by stretching and rotating the fibers along the fused length thereof as taught in U.S. Pat. No. 4,426,215. Also, a portion of the cladding is sometimes removed by etching or grinding to decrease the intercore distance as taught in U.S. Pat. No. 4,449,781. These processes are labor intensive and do not always result in the formation of couplers exhibiting the predetermined desired coupling characteristics. This latter mentioned disadvantage is particularly apparent in the manufacture of certain single-mode couplers wherein the coupling core sections are to remain parallel to ensure that the propagation constants are matched.
Whereas most couplers are made by applying heat directly to the fibers to be joined, U.S. Pat. No. 3,579,316 teaches a method wherein the fibers are first inserted into a capillary tube where the ends may overlap. The capillary tube is formed of a glass having a refractive index lower than that of the fiber cladding material. Heat is applied to the capillary tube in the vicinity of fiber overlap and the tube is stretched until the diameter thereof approximates that of the original fibers. The original cores of the pulled-out part become vanishingly small, their stretched diameters being only about 1/100 the original diameters. The cladding of the original fiber becomes the core of the coupling section.
The disadvantages of the method of U.S. Pat. No. 3,579,316 are as follows. The method is costly since fibers must be inserted into a capillary tube for each coupler. Also, the reduction in core diameter to 1/100 of the original diameter results in a lossy coupler, since the original cladding takes the place of the vanished cores. In the region of the coupler where the fiber cores taper from their "vanishingly small" size to their full size, an insufficient amount of the power can transfer from the cladding back to the core. Furthermore, it is difficult to maintain the cores straight and parallel to one another when fibers are inserted into a tube which is then stretched. Such nonlinear coupler cores can lead to decreased coupling efficiency in single-mode couplers.
Control of the excess device loss is a problem with prior art devices. Variability of mode power in the split waveguides exists due to fabrication technique variations. In some devices, bifurcation of the input and output fiber ports is difficult to accomplish. Also many of the prior art fabrication steps are highly labor intensive and are therefore a major cost source.